1. Field of the Invention
The present invention relates to severe plastic deformation of metals and alloys to control their structure and properties.
2. Description of the Prior Art
It is known in the art that severe plastic deformation performed by simple shear results in refinement of grain structures to the sub-micron, sometimes to nano scale. That leads to significant improvements in many physical and mechanical properties such as strength, ductility, fatigue, corrosion resistance, super plasticity, etc. Different processing methods were developed for intensive plastic deformation. Most of them are restricted by small sample sizes or soft materials and are used as a laboratory tool: high pressure torsion (P. W. Bridgeman, Studies in Large Plastic Flow and Fracture, McGrill, New York, 1952), cyclic extrusion-compression (A. Korbel, M. Richert, J. Richert, in: Second RISO International Symposium on Metallurgical Science, 1981, p. 485), repetitive corrugation and straightening (U.S. Pat. No. 6,197,129). Some techniques allow processing of sufficiently large billets and have potentials for industrial applications: equal channel angular extrusion (ECAE) (Invention Certificate of the USSR No 575892, 1974), accumulative roll-bonding (Y. Saito, N. Tsuji, H. Utsonomiya, T. Sakai and R. G. Hong, “Scripta Materialia”, 39, 1998, p. 1221), twist-extrusion (J. Beigelzimer, D. Orlov and V. Varyhin, in: “Ultrafine Grained Materials-II”, 2002, p. 297) and multi-directional forging (U.S. Pat. No. 6,422,090). Equal channel angular extrusion is considered the most promising candidate for practical applications and was used in many patents (see U.S. Pat. Nos. 5,400,633; 5,513,512; 5,600,989; 5,826,456; 5,850,755; 5,904,062). However, all these techniques are characterized by a few important disadvantages. As effective strains per pass are usually less than ε≦1 whereas accumulated strains for structure refinement ranges from ε=6 to ε=12, a large number of processing steps or passes should be used. Each pass requires billet preparation, preheating and lubrication. In result, such processing is time and labor consuming with a high product cost. Also, only simple billet shapes like short bars, rods or plates can be fabricated. In most cases, their conversion to final products presents additional problems with the increase in the cost. Therefore, these techniques are effective only for special applications. For example, the only reported commercialization of equal channel angular extrusion relates to sputtering targets in electronic and semiconductor industries (U.S. Pat. Nos. 5,590,389; 6,569,270).
It is very desirable to develop a cost effective industrial method for fabrication of complicated shapes like long extrusions with ultra-fine grained structures. These products may have numerous applications as structural materials in automotive, transportation, aero-space and other industries. However, the only known method in the art for such products is superplastic extrusion (see U.S. Pat. No. 5,620,537). This method comprises two step processing: (i) equal channel angular extrusion to prepare ultra-fine structures and (ii) superplastic extrusion. The first step conserves the above mentioned disadvantages of multi-pass ECAE. The second step should be realized with very low strain rates or high temperatures that leads to low productivity and degradation of the material structure and properties. Therefore, the known method does not provide evident technical benefits and did not find practical applications. The present invention is intended to resolve all these and other problems.